This invention relates to continuous filament polyester yarns having a low degree of crystallinity made by a high speed melt spinning process at controlled withdrawal speeds.
It has long been known that polymeric filaments, such as polyesters and polyamides, can be prepared directly, i.e., in the as-spun condition, without any need for drawing, by spinning at high speeds of the order of 5 km/min or more. This was first disclosed by Hebeler in U.S. Pat. No. 2,604,667 for polyesters, and by Bowling in U.S. Pat. No. 2,957,747 for polyamides. There has been increased interest in the last 10 years, as shown by the number of patent specifications disclosing methods of melt-spinning at these high spinning speeds.
Frankfort et al. in U.S. Pat. Nos. 4,134,882 and 4,195,051 disclose new uniform polyester filaments and continuous filament yarns of enhanced dyeability, low boil-off shrinkage and good thermal stability, prepared by spinning and winding directly at withdrawal speeds of 5 km/min or more. The highest speed exemplified is 8000 ypm. The withdrawal speed is the speed of the first driven roll wrapped (at least partially) by the filaments, i.e., the feed roll. When uniform polymeric filaments are desired, such as are suitable for continuous filament yarns, for example, it is essential to use a roll or equivalent positive means, driven at a constant controlled speed to withdraw the filaments, as opposed to an air jet ejector. The latter is satisfactory for some uses, such as non-woven products, but does not produce filaments that are sufficiently uniform for use as continuous filament yarns for most purposes.
Vassilatos in U.S. Pat. No. 4,425,293 discloses an oriented amorphous polyethylene terephthalate feed yarn for false-twist texturing prepared by spinning polyethylene terephthalate at a speed of at least 5000 m/min and quenching in a liquid bath to provide filaments having a boil off shrinkage (BOS) of at least 45% and no detectable crystallinity as measured by customary X-ray diffraction procedures. The liquid quenched yarn produced in U.S. Pat. No. 4,425,493 exhibits a rather low elongation to break, possibly attributable to the rapid quenching which introduces a large skin/core effect. By skin/core effect we refer to greater molecular orientation at the exterior or skin of the fiber than that orientation of the inner core. Such an effect is more pronounced when an effective quenching medium such as water is used rather than air. Upon loading, fibers with pronounced skin/core experience significant radial stress differences which lead to premature breaking. The production at ultra high speed, above 5000 m/min of a low crystallinity yarn with a higher elongation to break would be highly desirable.